The police are already hunting for 18 prisoners who are "unlawfully at large" after being released among the 1,700 let out during the first week of the government's new emergency scheme to cope with jail overcrowding, the justice ministry said yesterday.

The 18 are part of a group of 30 prisoners who will be sent back to jail because they have either reoffended, failed to turn up at their listed accommodation address or failed to contact their probation officer.

The junior justice minister Lord Hunt of Kings Heath told parliament yesterday that there were three further prisoners who had either been released a week too early or let out without proper authorisation. Two of them are now back in custody.

The official figures show that 1,701 prisoners were released early between June 29 and July 5 in the first week of operation of the "end of custody licence" scheme. Those serving sentences of between four weeks and four years for crimes excluding serious violent and sexual offences are let out 18 days early under the scheme.

The introduction of the scheme has cut the overall prison population in England and Wales from 81,040 to 79,767 in the last fortnight.

Lord Hunt said all prisoners released under the scheme were liable to recall if they were reported to have misbehaved during the period of the licence. The figures show that of the 30 recalled to prison so far, six had reoffended, six failed to turn up at their approved address, 11 had failed to get in touch with their probation officer, and seven had been involved in poor behaviour. Some had been recalled for more than one reason.